The Triplet Code (AQA A Level Biology): Revision Note

The triplet code

• The DNA nucleotide base code found within a gene is a three-letter, or triplet, code

• Each triplet codes for one amino acid

• There are four bases (A, G, C, T), so there are 64 different triplets possible (4³), yet there are only 20 amino acids that commonly occur in biological proteins

  • This results in multiple triplets coding for the same amino acids; thus, the code is said to be degenerate (this can limit the effect of mutations)

• Some triplets code for start (TAC – methionine) and stop signals

  • These signals tell the cell where individual genes start and stop

  • This ensures the cell

    • reads the DNA correctly as the code is non-overlapping

    • can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly

• The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)

• This means that the same triplet codes for the same amino acids are found in all living organisms